A method for vulcanizing a green tire is known that includes inserting a vulcanization bladder into a green tire placed in a mold, introducing steam to be a heating medium into the vulcanization bladder, and then introducing nitrogen gas (inert gas) to be a pressurizing medium into the same. The vulcanization time is determined on the basis of a portion requiring the longest vulcanization time (a portion where the vulcanization speed is slowest). For example, an unvulcanized rubber containing a large amount of silica requires a long vulcanization time. Thus, when a green tire composed of a tread rubber containing a large amount of silica is vulcanized, the vulcanization time is determined on the basis of the tread portion, and over-vulcanization occurs more readily in portions other than the tread portion as the vulcanization is performed at higher temperature. The greater the over-vulcanization, the more adversely affected the physical properties of the vulcanized rubber are. It is thus desirable to prevent over-vulcanization to the utmost.
A low internal pressure applied by the vulcanization bladder to the green tire in the initial stage of vulcanization causes the unvulcanized rubber to flow insufficiently and may thus cause vulcanization defects. Such vulcanization defects are significant especially in vulcanizing a tire having high rigidity.
A vulcanization device has been proposed that, for example, preliminarily mixes steam and inert gas in a tank and introduces the resultant gas mixture into a vulcanization bladder (see Japanese Unexamined Patent Application Publication No. H09-76239A). With this vulcanization device, introducing the gas mixture in the tank into the vulcanization bladder can increase internal pressure applied to a green tire in the initial stage of vulcanization. Moreover, the vulcanization time can be shortened to some extent. Unfortunately, this device requires a tank for preliminarily mixing steam and inert gas, resulting in an increase in equipment size.